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Physical Chemistry October 2010 Vol.55 No.30: 3472–3478

doi: 10.1007/s11434-010-4054-y

One-pot synthesis of PVP-coated Ni0.6Fe2.4O4 nanocrystals

LU XianYong1,2, NIU Mu1, YANG ChunHui1, YI LuoXin1, QIAO RuiRui1, DU MeiHong3 & GAO MingYuan1*

1 Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China;

2 School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100191, China;

3 Beijing Center for Physical and Chemical Analysis, Beijing 100089, China

Received January 4, 2010; accepted April 27, 2010

Novel poly(N-vinyl-2-pyrrolidone) (PVP)-coated nickel ferrite nanocrystals were prepared by simultaneously pyrolyzing nickel(II) acetylacetonate (Ni(acac)2) and iron(III) acetylacetonate (Fe(acac)3) in N-vinyl-2-pyrrolidone (NVP). The PVP coating was formed in situ through polymerization of NVP. The crystalline structure of the resultant nickel ferrite was analyzed by high-resolution transmission electron microscopy, electron diffraction patterns, and powder X-ray diffraction. In addition, the valence state of Ni and the metal contents of Ni and Fe in different valence states were analyzed by X-ray photoelectron spectros-copy (XPS), atomic absorption and the phenanthroline method. The surface coating layer of PVP and its binding states were characterized by Fourier transform infrared spectroscopy in combination with XPS. Colloidal stability experiments revealed that the nanocrystals could be dispersed well in both phosphate-buffered saline and Dulbecco’s Modified Eagle Medium.

magnetic nanocrystals, thermal decomposition, N-vinyl-2-pyrrolidone, colloidal stability

Citation: Lu X Y, Niu M, Yang C H, et al. One-pot synthesis of PVP-coated Ni0.6Fe2.4O4 nanocrystals. Chinese Sci Bull, 2010, 55: 3472−3478, doi: 10.1007/s11434-010-4054-y

Nanocrystals of magnetic ferrites, i.e. MxFe3–xO4 (M = Fe, Mn, Co, Ni, Zn; 0≤ x ≤1), have recently received extensive attention because of their potential application in hyper-thermia treatment [1–3], magnetic data storage [4], catalysis [5], magnetic resonance imaging (MRI) as contrast agents [6–10], and microwave devices [11]. As a soft magnetic n-type semiconducting material with a fully inverse spinel structure, nickel ferrite is an important member of the spinel family [12–14]. The special physical properties of low magnetic coercivity and high electrical resistivity make nickel ferrite an excellent core material for power trans-formers in electronic and telecommunication applications [15]. Different synthetic methods such as mechanosynthesis [16], hydrothermal synthesis [17], coprecipitation [18], combustion synthesis [19], sol-gel methods [20], microwave processing [21], and thermal decomposition [10,22], have been used so far to produce nickel ferrite nanocrystals. *Corresponding author (email: gaomy@iccas.ac.cn)

Among these methods, thermal decomposition of organic metal precursors in high boiling point solvents has been demonstrated as a reliable route for preparing ferrite nano- crystals with uniform size, a high degree of crystallinity, and a clearly defined phase structure [10,23,24].

A thermal decomposition reaction system typically con-tains three components: a high boiling point solvent, an or- ganic metal precursor and a surface capping agent. For ex- ample, Sun et al. [24] reported the synthesis of monodis-perse ferrite nanoparticles using benzyl ether as a solvent, metal acetylacetonate complexes as an organic metal pre-cursor, and oleic acid and oleylamine as surface ligands. Cheon et al. reported the preparation of ferrite nanoparticles by a similar method, which involved a high-temperature reaction between a divalent metal chloride (MCl2, M=Mn, Fe, Co, or Ni) and iron tris-2,4-pentadionate in the presence of oleic acid and oleylamine as surfactants [9]. Bao et al. [22] also succeeded in preparing high quality ferrite nanoparticles by thermolysis of a 2+M 3+

2Fe -oleate complex

LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30 3473

(M=Fe, Co, Ni) dissolved in 1-octadecene at 300°C in the presence of oleic acid. Although short chain fatty acids and amines have also been found useful for controlling the syn-thesis of ferrite nanoparticles [25], they possess quite simi-lar surface properties to the resulting nanoparticles. There-fore, the surface functionalization of ferrite nanoparticles using polymers containing various types of functional groups appeared as an alternative way of producing mag-netic nanoparticles with different surface properties, which greatly broadened their potential applications [26,27]. In general, the polymer coating can be realized either by (1) pyrolyzing the metal precursors in the presence of polymers containing functional groups which can anchor onto the surface of the resultant particles [6,7], or (2) performing thermal decomposition in a coordinating solvent with po-lymerizable properties [28]. Pyrolysis of ferric triacetylace-tonate (Fe(acac)3) in N-vinyl-2-pyrrolidone (NVP) is a good example of the latter approach, producing poly(N-vinyl- 2-pyrrolidone)-coated magnetic nanoparticles with a greatly simplified reaction system. In this reaction system, NVP was not only used as a coordinating solvent, but also as a radical monomer for the in situ formation of a surface coating of poly(N-vinyl-2-pyrrolidone) (PVP). Fe(acac)3 acted as both an iron precursor and as a polymerization initiator. The re-sulting PVP coating enabled the magnetite nanocrystals obtained to be dispersed in ten different types of organic solvents as well as in aqueous solutions of different pH with very little variation in the hydrodynamic sizes of the parti-cles [28]. Such dispersibility in liquid media is greatly de-sirable for various applications of nanoparticles.

Following on from our previous investigations, the syn-thetic method mentioned above was extended further to prepare PVP-coated nickel ferrite nanocrystals by simulta-neously pyrolyzing Ni(acac)2 and Fe(acac)3 in NVP. Herein, we report the preparation and crystalline structure of the resultant nickel ferrite nanocrystals, as well as discussing the reaction mechanism.

1 Materials and methods

1.1 Materials

Fe(acac)3 and Ni(acac)2 were purchased from Aldrich (14024-18-1, 97%; 3264-82-2, 95%, respectively) and used after recrystallizing twice. Medical grade NVP was obtained from Jiaozuo Meida Fine Chemical Co., Ltd and purified by distillation under reduced pressure in the presence of phe-nothiazine. All other chemicals were of analytical grade and were used as received.

1.2 Synthesis of nickel ferrite nanocrystals

Nickel ferrite nanocrystals were synthesized by pyrolyzing Fe(acac)3 and Ni(acac)2 in NVP at 200°C. In a typical preparation, Fe(acac)3 and Ni(acac)2 were first dissolved in

NVP with a molar ratio of 2:1. The total concentration of metal (Fe, Ni) precursors was 0.06 mol/L. The solution was purged with argon for 30 minutes at room temperature to remove oxygen, and subsequently heated to 200°C under argon. Aliquots were extracted from the reaction mixture at 1, 2, 4 and 6 h, and denoted as samples A, B, C and D, re-spectively. The nanocrystals were purified as follows. First, equal volumes of ethanol were introduced into the aliquots extracted at different times. Five times this volume of di-ethyl ether was then added to induce precipitation. The re-sulting black precipitates were then isolated using a perma-nent magnet. The nanocrystal samples were obtained by repeating this precipitation procedure three times.

To reveal that nickel ferrite nanoparticles had formed, the thermal decomposition of Ni(acac)2 in NVP was also inves-tigated. The reaction procedure was generally the same as that for the nickel ferrite nanocrystals. First, Ni(acac)2 was dissolved in NVP to give a 0.04 mol/L solution. The resulting light-green transparent solution was purged with argon gas for 30 min at room temperature to remove oxygen, and then heated to 200°C. The color of the reaction mixture turned from light-green to black during the heating process. The reaction typically finished within 1 h at 200°C. The resulting nanoparticles were purified by the procedure described above.

1.3 Characterization

Transmission electron microscope (TEM) and electron dif-fraction (ED) measurements were performed on a JEM- 100CXII electron microscope at an acceleration voltage of 100 kV. High-resolution TEM (HRTEM) images were taken on a JEM-2100F microscope at an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDXS) was carried out using a GENESIS system (EDAX Inc.) attached to a TEM. The content of Fe and Ni in the nanoparticle samples were analyzed on a PerkinElmer AAnalyst™ 800 high-performance atomic absorption spectrometer (AAS) equipped with a high performance burner system. Elemental Fe and Ni were detected at 248.3 nm and 232.0 nm, respec-tively, with single element hollow cathode lamps. The spec-tral bandwidth for all of the element detections was set as 0.7 nm. The molar ratio of Fe(III) and Fe(II) was deter-mined by the phenanthroline method [29]. Powder X-ray diffraction (XRD) measurements were preformed on a Ri-gaku D/Max-2500 diffractometer with Cu Kα1 radiation (λ = 1.54056 Å). The hydrodynamic diameter of the magnetic nanocrystals was determined with a Malvern Zetasizer (Nano ZS). Fourier transform infrared spectroscopy (FTIR) was performed on a Bruker EQUINOX55 FTIR spectrome-ter under ambient conditions. X-ray photoelectron spec-troscopy (XPS) data were obtained with an ES-CALab220i-XL electron spectrometer from VG Scientific using 300 W Mg Kα radiation. The base pressure was about 3×10–9 mbar. Thermogravimetric analysis (TGA) was car-

3474 LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30

ried out on a PerkinElmer Pyris 1 analyzer. The temperature range was set between 20 and 850°C with a temperature increase rate of 10°C/min under a nitrogen atmosphere. The magnetic properties of the samples were characterized by a vibrating sample magnetometer (VSM, JDM-13, China).

2 Results

Figure 1 shows TEM images of samples A-D. The four samples generally contain quasispherical nanoparticles with average diameters that increase slightly with reaction time, i.e. 4.4±0.9 nm for sample A, 5.1±1.2 nm for sample B, 6.7±1.4 nm for sample C, and 6.7±1.7 nm for sample D. The HRTEM results shown in Figure 2 demonstrate that the quasispherical particles are single crystals with crystal planes that are consistent with those of Ni0.6Fe2.4O4. The electron diffraction results presented in Table 1 reveal that the d-spacing values of the nanocrystals closely match those of the JCPDS card for Ni0.6Fe2.4O4 (No. 87-2338), which is consistent with the lattice analysis in Figure 2. The crystal- line structure of the nanoparticles was further verified by XRD. The results shown in Figure 3 provide strong evi-dence that the nanoparticles are composed of Ni0.6Fe2.4O4.

The metal compositions in the samples were analyzed by energy-dispersive X-ray (EDX) spectroscopy. The experi-mental results shown in Figure S1 of the Supporting Infor-mation (SI) demonstrate that the atomic ratio of Ni:Fe in

sample D is 1:4.0, consistent with the stoichiometric ratio

of Fe to Ni in Ni0.6Fe2.4O4. The molar ratios of Ni:Fe in the

other samples, determined by AAS, were 1:3.7 for sample

A, 1:4.1 for sample B, and 1:3.7 for sample C, which are

also quite close to the theoretical value of 1:4. The presence of Ni(II) in the nanoparticles was further

verified by XPS analysis. All of the binding energies for Ni 2p, Ni 3p, O 1s, and N 1s were calibrated to C 1s (284.8 eV) of adventitious carbon as a reference. The spectra of Ni 2p and Ni 3p (Figure 4) were fitted using a Shirley-type back-ground. The binding energy of Ni 2p and Ni 3p are 854.5 eV and 67.1 eV, respectively, which demonstrates that Ni mainly exists in the crystal lattice in the form of divalent ions [30–32]. Based on the XPS result, the molar ratio of Ni(II) to Fe(II) in the particles was further determined by AAS in combination with the phenanthroline method to differentiate Fe(II) from Fe(III). The experimentally ob-tained ratio was 2.2:1, which is close to the theoretical

value of 1.5:1 for Ni0.6Fe2.4O4 (Ni2+0.6Fe2+

0.4Fe3+2O

2–4).

Sample D was also characterized by FTIR (Figure 5). Compared with the FTIR spectrum of NVP which shows vibrational peaks of the vinyl group at 1630 cm–1 and 983 cm–1, the spectrum of sample D lacks these signals, which suggests that NVP is polymerized during the reaction to form PVP [33]. The vibrational band from the carbonyl group shifts from 1680 to 1657 cm–1 after the reaction. This

implies that the resultant PVP is chemically anchored onto the surface of the Ni0.6Fe2.4O4 nanocrystals via the carbonyl group of PVP [28,33,34].

The magnetic properties of the as-synthesized nickel fer-rite nanocrystals were studied with a VSM. The room tem-perature magnetization curve shown in Figure 6 demon-strates that the nanoparticles are superparamagnetic with a

Figure 1 TEM images of sample A (a), sample B (b), sample C (c) and sample D (d).

LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30 3475

Figure 2 HRTEM images of sample A (a) and sample D (b) overlaid with identification of crystal planes of Ni0.6Fe2.4O4.

Table 1 The d-spacing values obtained based on the electron diffraction patterns of samples A–D, together with the data for Ni0.6Fe2.4O4 from JCPDS card no. 87-2338

Sample A Sample B Sample C Sample D Ni0.6Fe2.4O4 hkl

2.9478 2.9843 2.9627 2.9670 2.9559 220

2.5275 2.5590 2.5290 2.5320 2.5208 311

2.0965 2.1029 2.0943 2.0993 2.0902 400

1.7146 1.7211 1.7203 1.7146 1.7066 422

1.6082 1.6120 1.6152 1.6158 1.6090 511

1.4776 1.4744 1.4900 1.4845 1.4779 440

1.2747 1.2863 1.2867 1.2807 1.2750 533

saturation magnetization of 31.1 emu/g. Taking the organic content of sample D (48.8%) determined by TGA into con-sideration, the saturation magnetization value of pure Ni0.6Fe2.4O4 in sample D is around 60.9 emu/g.

Figure 3 Powder X-ray diffractogram of sample D. The line patterns shown on the bottom are standard data for Ni0.6Fe2.4O4 from JCPDS card 87-2338.

Figure 4 X-ray photoelectron spectrum of sample D (a), and high resolu-tion spectra of Ni 2p (b) and Ni 3p (c).

3 Discussion

As demonstrated above, the preparation used led to Ni0.6Fe2.4O4 nanocrystals independent of the reaction time between 1 and 6 h. The atomic ratio of Ni/Fe in the resultant nanocrystals is dissimilar to the molar ratio of the reactants,

3476 LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30

Figure 5 FTIR spectra of NVP (dashed line) and sample D (solid line).

Figure 6 Room temperature magnetization curve of sample D.

i.e. 1:2, from which NiFe2O4 was expected according to the literature [9,22]. To further reveal why Ni0.6Fe2.4O4 formed instead of NiFe2O4, a solution of Ni(acac)2 with a 0.04 mol/L concentration was pyrolyzed in NVP. The thermoly-sis of Ni(acac)2 gave rise to a large color change from green to black within the first 10 min of reaction at 200°C. Such a color change was not observed when Fe(acac)3 was present in the reaction system. Further TEM and electron diffraction characterization of the black sample collected after 1 h reac-tion demonstrated that the resultant particles are different from those prepared in the presence of Fe(acac)3. As shown in Figure 7, the average size of the particles is 45.4 nm. The d-spacing values calculated from the electron diffraction patterns suggest that the particles are cubic nickel according to standard data from JCPDS card (No. 04-0850). In con-trast, nickel particles were not obtained by pyrolyzing Ni(acac)2 in phenyl ether under the same experimental con-ditions, implying that NVP can reduce Ni(II) at high tem-perature to form metallic Ni.

Although the details of the thermal decomposition be-havior of the metal acetylacetonate complexes in NVP is difficult to observe at high temperature, the experimental results suggest that NVP might reduce Ni(acac)2 to Ni(II) more easily than it reduces Fe(acac)3 to Fe(II). Therefore, a possible mechanism for the formation of Ni0.6Fe2.4O4 (Ni2+

0.6Fe2+0.4Fe3+

2O2–

4) is proposed as follows. In the early stages of the reaction, Ni(acac)2 is quickly decomposed and reduced to form elemental nickel which subsequently reacts with Fe(III), giving rise to both Ni(II) and Fe(II) while the

metal oxide particles are formed. As Ni(II) and Fe(II) pos-sess comparable ionic radii [35], they can both enter the crystal lattice, so Ni0.6Fe2.4O4 is formed rather than NiFe2O4. It is known that Fe(III) can partly be reduced when Fe(acac)3 is pyrolyzed in either 2-pyrolidone [36,37] or N-vinyl-2-pyrolidone [28]. The possible formation of Fe(II) by reaction of elemental Ni with Fe(III) in the early stage of the reaction is more favorable because it is supported by the fact that Ni particles were not found in the reaction system containing both Ni(acac)2 and Fe(acac)3.

The novel Ni0.6Fe2.4O4 nanocrystals possess similar properties to magnetite, making them attractive for bio-medical applications [6–10]. Solubility and colloidal stabil-ity in water are basic requirements for bioapplications of magnetic nanoparticles [23]. Activity in aqueous media around neutral pH is an important condition for in vivo ap-plications to coincide with physiological pH (7.35–7.45 for human blood). Nevertheless, satisfying colloidal stability at neutral pH is not enough for magnetic nanoparticles; bio-medical applications require physiological conditions with a 0.17 mol/L ionic strength. Phosphate-buffered saline

Figure 7 TEM images (a) and electron diffraction patterns (b) of the nanoparticles obtained by pyrolyzing Ni(acac)2 in NVP. The diffraction rings are labeled with the Miller indices of cubic Ni (JCPDS card No. 04-0850).

LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30 3477

Figure 8 Hydrodynamic size distribution profiles of sample D in pure water, PBS and DMEM (Inset: photographs of sample D in PBS and DMEM).

(PBS) can simulate above-mentioned conditions because its pH and ionic concentration are similar to body fluids. For actual physiological conditions, a variety of proteins should also be taken into consideration besides the pH and ionic strength. Dulbecco’s Modified Eagle Medium (DMEM) with a high concentration of glucose is widely employed for supporting the growth of a broad range of mammalian cell lines and can mimic physiological conditions well. There-fore, the colloidal stability of the nickel ferrite nanocrystals was investigated in different types of biological media such as PBS and cell culture media. The hydrodynamic size of sample D in pure water, 1 mol/L PBS buffer and DMEM cell culture medium was determined to be 17.3, 19.0 and 20.6 nm, respectively. Most importantly, the PVP-coated Ni0.6Fe2.4O4 nanocrystals do not agglomerate in either PBS or DMEM cell culture medium (Figure 8), which allows them to be used further in some bioapplications, for example, magnetic labeling of cells.

The excellent water solubility and good colloidal stability of the as-prepared Ni0.6Fe2.4O4 nanocrystals in different so-lutions that mimic physiological conditions can be attrib-uted to PVP binding to the particle surface as was found in the FTIR investigations. To provide more details about the binding of PVP, the XPS signals of N 1s and O 1s were fitted by multiple Gaussian functions with an equal full width at half-maximum using a Shirley-type background. The results are shown in Figure 9. The binding energy of the N 1s signal of sample D is similar to that of pure PVP [38]. However, the fitting results of the O 1s signal suggest that the O 1s signal is a convolution of three peaks at 529.8, 531.1, and 532.2 eV. According to reference data, these three peaks can be assigned to O from Ni0.6Fe2.4O4 [31], O from PVP [38], and O from PVP coordinating with Ni0.6Fe2.4O4

nanocrystals, respectively [28]. Based on this analysis, the ratio between the coordinating and non-coordinating PVP segments is estimated to be around 1:3, which explains the excellent colloidal stability of the resultant nanocrystals in PBS and DMEM aqueous solutions. This colloidal stability

Figure 9 X-ray photoelectron spectra of N 1s (a) and O 1s (b) signals recorded for sample D. The dashed lines are calculated spectra.

arises from the free PVP segments, while the coordinating segments of PVP mainly serve as stabilizing agents, pre-venting the nanocrystals from uncontrolled growth like the conventional surfactants commonly used in nanocrystal synthesis.

4 Conclusions

In summary, novel PVP-coated nickel ferrite (Ni0.6Fe2.4O4) nanocrystals were prepared in a one-pot reaction by simul-taneously decomposing Ni(acac)2 and Fe(acac)3 in NVP at 200°C. In this preparation, NVP was used as a high boiling point coordinating solvent. The polymerizable nature of NVP allowed PVP to be formed in situ as a surface coating on the nanocrystals. Systematic investigations were per-formed to identify the crystalline structure and formation mechanism of this new type of nanocrystal. FTIR and XPS investigations revealed that PVP coordinates on the nickel ferrite nanoparticle surface via carbonyl groups. As only one quarter of the PVP segments coordinate to the nanocrystals, the remaining PVP segments offer the nanoparticles excel-lent colloidal stability in both PBS buffer and DMEM cell culture media, making the particles potentially useful in magnetic cell labeling applications.

This work was supported by the National Natural Science Foundation of China (20640430564 and 20820102035), the Knowledge Innovation Pro-gram of the Chinese Academy of Sciences (KJCX2-YW-M15) and the Min-istry of Health (2008ZX10001-014).

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Supporting Information

Table S1 TEM image of sample D (a) and EDX spectrum (b) recorded from the particles presented in the black square. The atomic percentages of Fe and Ni elements in given area are provided in the table inserted in frame (b).

The supporting information is available online at csb.scichina.com and www.springerlink.com. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains en-tirely with the authors.

LU XianYong, et al. Chinese Sci Bull October (2010) Vol.55 No.30

Table S1 TEM image of sample D (a) and EDX spectrum (b) recorded from the particles presented in the black square. The atomic percentages of Fe and Ni elements in given area are provided in the table inserted in frame (b).